I think Zingano has the knockout power, we just didn't get to see her test Rousey's chin because she was too hyped up and ran right into a throw. She is a very strong striker, but not on Holm's level.
That said, Holm has not looked even close to as good she did against Rousey in her prior UFC fights. Having only seen her in those, I thought she would get destroyed by Rousey. Raquel Pennington took her to a split decision, and she is not very good. Then her second fight was against someone I had never heard of and underwhelmed again. She was a beast this fight.
I'm not convinced that it will go the same if they fight again. I don't think Holm's grappling would hold up against Rousey if that is the fight Ronda brings. If Rousey comes in super aggressive, ready to redeem herself on her feet she will get knocked out again. You're 100% correct about the holes in her standup game. If she dirties the fight up next time, I think her grappling will win it for her.
Regardless, Holm winning was a great thing for the division in my opinion. Maybe not publicity-wise, but for competitive balance. If Ronda has a year absence, I'd love to see Holm fight Tate. I think that would be a great fight, unless Tate is overconfident in her hands too.